The proposed project is divided into two sections. The first proposes to determine the amino acid sequence of precursor proteins translated from messenger RNA in vitro with 14C or 3H amino acids. The radiolabeled proteins will be purified by polyacrylamide gel electrophoresis and quantitatively sequenced. The sequences will provide information on the intracellular function of the precursors and on the arrangement of immunoglobulin genes. The second part of the proposal deals with the purification and chemical characterization of the Fc receptor from mouse mastocytoma cells. The radiolabeled Fc receptor will be isolated with antibody-antigen complexes and purified and characterized on polyacrylamide gels. Comparisons of the Fc receptor isolated with different classes of Ig and different cell types will be done to determine if more than one species of protein is involved.